The present invention relates to a piezoelectric vibrator manufacturing method for manufacturing a piezoelectric vibrator of a surface mount device (SMD) type in which the piezoelectric vibrator is encapsulated in a cavity formed between bonded two substrates, a piezoelectric vibrator manufactured in this manufacturing method, and an oscillator, electronic apparatus, and a radio clock having this piezoelectric vibrator.
In recent years, a piezoelectric vibrator using crystal or the like is used as a time instance source, a timing source for control signals, and a reference signal source in mobile phone sets or portable information terminal devices. There are provided various types of such piezoelectric vibrators, and a piezoelectric vibrator of a surface mount device (SMD, Surface Mount Device) type is known as one of these piezoelectric vibrators. As the piezoelectric vibrator of this type, a three-layer structure type in which a base substrate and a lid substrate are bonded to a piezoelectric substrate formed with a piezoelectric vibrating strip thereon so as to interpose the same therebetween from above and below is generally known. In this case, the piezoelectric vibrating strip is stored in a cavity (sealed chamber) formed between the base substrate and the lid substrate.
Also, in recent years, there is also developed a two-layer structure type instead of the three-layer structure type described above. A piezoelectric vibrator of this type has a two-layer structure having a base substrate and a lid substrate bonded directly to each other, and a piezoelectric vibrating strip is stored in a cavity formed between the both substrates. The piezoelectric vibrator of the two-layer structure type is superior in that reduction in thickness is achieved, for example, in comparison with those having the three-layer structure, and is preferably used.
Incidentally, an electrode film for electrically connecting an external electrode formed on the base substrate and the piezoelectric vibrating strip stored in the cavity is formed between the base substrate and the lid substrate (bonded surface). Also, the base substrate and the lid substrate are bonded via anodic bonding using this electrode film. In other words, the electrode film is used as a bonding film for applying a voltage at the time of the anodic bonding. Because of this anodic bonding, the piezoelectric vibrating strip is in a state of being reliably encapsulated within the cavity.
As the electrode film, for example, a conductive film such as aluminum or chrome, or a semiconductor film such as silicon is used. Insulative substrates such as glass (soda lime glass) are used as the base substrate and the lid substrate.
The piezoelectric vibrator is generally manufactured as follows.
First of all, a plurality of depressions which serve as the cavities are formed on a base substrate wafer which is formed into the base substrates, and the electrode film which also serves as the bonding film used at the time of the anodic bonding is patterned from the depressions to the bonded surface. Then, a plurality of depressions which serve as the cavities are formed on a lid substrate wafer which is formed into the lid substrates. Then, after having mounted the piezoelectric vibrating strips in the depressions formed on the base substrate wafer, the lid substrate wafer is superimposed on the bonded surface of the base substrate wafer. Then, the both wafers are bonded via the anodic bonding by utilizing the electrode film to form a piezoelectric vibrator wafer member. Thereafter, by cutting the piezoelectric vibrator wafer member with a dicing blade into small pieces in a grid-like pattern, individual piezoelectric vibrators are obtained. Then, finally, by forming the external electrode on the base substrate, the piezoelectric vibrator of the surface mount device type is manufactured.
Incidentally, the piezoelectric vibrator is generally desired to have an equivalent resistance value (effective resistance value, Re) reduced to a low value. The piezoelectric vibrator having the low equivalent resistance value is able to vibrate the piezoelectric vibrating strip at a low power, and hence the piezoelectric vibrator having high energy efficiency is achieved.
As a general method of reducing the equivalent resistance value, a method of bringing the interior of the cavity in which the piezoelectric vibrating strip is encapsulated near under vacuum is known. In other words, by bringing the interior of the cavity near under the vacuum, a series resonance resistance value (R1) which is in proportion to the equivalent resistance value can be lowered. In other words, the degree of vacuum in the interiors of the cavity and the series resonance resistance value are in inverse proportion.
Then, as a method of adjusting the degree of vacuum in order to bring the interior of the cavity near under the vacuum, a method of storing a getter material such as aluminum in the cavity, and activating the getter material by irradiating the same with a laser from outside by a gettering adjusting machine (gettering adjustment) is known. In this method, since oxygen generated at the time of the anodic bonding can be absorbed by the getter material in an activated state, the interior of the cavity is brought near under the vacuum.
As the method of bringing the interior of the cavity near under the vacuum, a method of arranging a moisture adsorbent in the cavity is also known (for example, see Patent Document 1). This method is a method applied when bonding the lid substrate and the base substrate with an adhesive agent instead of the anodic bonding, and enables the interior of the cavity to be brought near under the vacuum by adsorbing moisture entering from the outside into the cavity through the adhesive agent by the moisture adsorbent.
Incidentally, a nominal frequency is determined generally for the piezoelectric vibrator. This nominal frequency is a value which guarantees a frequency when a predetermined voltage is applied to the piezoelectric vibrating strip. In other words, the respective piezoelectric vibrating strips are needed to be adjusted in frequency so as to vibrate within a range of the nominal frequency when the voltage is applied.
Therefore, the frequency adjustment for the piezoelectric vibrating strips is performed. In general, as a method of the frequency adjustment, there are a coarse adjustment step performed after having manufactured the piezoelectric vibrating strip and a fine adjustment step performed after having encapsulated the piezoelectric vibrating strip in the cavity. Specifically, in the fine adjustment step, it is necessary to adjust the frequency of the piezoelectric vibrating strip adequately so as to cause the piezoelectric vibrating strip in a state of a product to vibrate within the range of the nominal frequency, and hence it is necessary to perform the frequency adjustment while measuring the frequency of the piezoelectric vibrating strip. In other words, it is necessary to vibrate the piezoelectric vibrating strip encapsulated in the cavity by applying a predetermined voltage thereto.    Patent Document: JP-A-56-98015